The present invention relates to an optical apparatus for an image pickup system such as a video camera or still camera, a projection system such as a projector or a rear-projection TV, and the like. More specifically, it relates to a light quantity-control device for regulating the quantity of light in association with the optical apparatus and a magnet rotor used as a driving power source for the light quantity-control device.
In general, a light quantity-control device incorporated in an optical apparatus, i.e., camera and projector devices, is widely used as a shutter device or a light-limiting device. Such devices have a frame substrate (bottom board) placed at a midway position in an optical path and a thin plate-shaped blade member openably mounted on the frame substrate, wherein the shutter device opens and closes the blade member thereby adjusting the quantity of light passing therethrough or cut the optical path.
The light-limiting device further operates to regulate, i.e., increasing and decreasing, the quantity of light. The blade member includes one or more blades, and is pivotably or slidably supported around an optical path aperture formed in the frame substrate. The one or more blades are driven by an electromagnetically-driving device operable to open and close the blades.
An electromagnetically-driving device may include a magnet rotor with a permanent magnet and a stator coil having an excitation coil for applying a rotation force to the rotor. An electromagnetically-driving device like this may provide a device, such as a camera device for cellular telephones, with reductions in size and weight, and power consumption. Such savings have been demanded as key technical issues, and various proposals for downsizing of the device have been made.
Patent Document 1, JP-A-2004-334038, discloses a rotor including a cylindrical permanent magnet integrated with a rotary shaft and supported rotatably with respect to a coil frame. A coil is wound on an external circumference of the coil frame, and an external circumference of the coil frame thus formed is shielded by a yoke.
However, the structure has the feature that as the driving device, and particularly its core part, become larger in diameter, the rotary shaft arranged in its center, the permanent magnet arranged around an external circumference of the rotary shaft, the coil frame and coil arranged around an external circumference of the permanent magnet, and the yoke arranged around an external circumference of the coil frame with the coil, likewise becomes larger.
The rotary shaft is press-fitted into a center shaft bore formed in the magnet, or is secured by an adhesive. According to Patent Document 1, a narrowed portion is formed in the rotary shaft along a diameter thereof and the adhesive is filled in the narrowed portion, thereby preventing the adhesive from flowing out. However, downsizing the device makes the diameter of the rotary shaft smaller, which poses a difficult problem in machining the rotary shaft.
Patent Document 2, JP-A-2002-049076, discloses a method of forming a magnetic pole using a strip of a soft magnetic material to lead the magnetism generated by an annular excitation coil to a hollow magnet rotor, wherein the magnet rotor and excitation coil are arrayed vertically in a direction of the shaft.
Patent Document 2 further discloses wherein the magnet rotor is formed in a hollow cylindrical shape and an inside yoke and an outside yoke are arranged in a central aperture portion of the rotor and in an outer peripheral portion, respectively, so that each of them surrounds the magnet of the rotor. The inside and outside yokes lead a magnetic force from a coil to the surrounding of the magnet. The location of the magnetic force is different from the location where the magnet rotor is located, thus generating the rotation force.
An electromagnetically-driving device like this has the structure in which a thin plate-shaped outside yoke is disposed around an external circumference of a hollow magnet rotor and a sleeve-like inside yoke is provided inside the magnet rotor. Therefore the device can be downsized by reducing the outer diameter of the device. In addition, the outer diameter of the magnet rotor can be made relatively larger, and therefore a larger rotation force can be achieved. (This is because a coil frame and coil are not provided around an external circumference of the rotor.)
As for a magnetic circuit constituted along a direction of the flow of magnetism, the outside yoke, magnet rotor and inside yoke, the gap between the outside yoke and magnet rotor and the gap between the magnet rotor and inside yoke can be minimized within the bounds of allowing the rotation of the rotor. Therefore, the magnetic circuit has the following feature: the permanence of the whole magnetic circuit is high enough to reduce the electric power consumption.
As disclosed by the magnet rotor in the above-described electromagnetically-driving device, the magnet is secured to the rotary shaft, and a driving arm is integrated with the rotary shaft. Production methods of integrating the driving arm, rotary shaft and magnet include the following. The first is a method of integrally molding the driving arm and rotary shaft out of a resin, for example, and then performing insert molding thereof when the magnet is shaped. The second is a method of securing the driving arm and rotary shaft, each of which has been integrally molded, to the magnet by an adhesive. The third is a method of press-fitting the rotary shaft into the shaft bore of the magnet thereby to secure the rotary shaft to the magnet.
As for the first method, i.e. the insert molding method, when the diameter of the rotary shaft is reduced to approximately 1 to 3 mm in order to downsize an electromagnetically-driving device, it becomes difficult to design a molding die. At the same time, it is difficult to handle a tiny molded article, and the skill and troublesome work are required for insert molding. In the second case, where the rotary shaft and driving arm are secured to the magnet by an adhesive, there is the problem that include, for example, outflow of the adhesive lading to a defective operation. Also, in the case of securing by press-fit, there is the problem of the magnet damaged at the time of press-fitting. In addition, there is the problem of difficulty of molding a rotary shaft which is made finer increasingly.
Furthermore, in the method disclosed by Patent Document 2, which allows the rotary shaft to be constructed so as to have a relatively larger diameter, the driving arm and rotary shaft are integrally molded and secured to the magnet. While the securing method is not disclosed in the document, problems as described above arise when a conventionally used securing method, e.g., press-fit, adhesive bonding, or insert molding, is adopted.
Therefore, with any conventional method, there have been the following problems in securing the rotary shaft to the magnet. The first problem is that the fragile magnet is often damaged during assemble in the case of the securing method by press-fit. The second problem is that outflow of the adhesive leads to a defective operation in the case of the securing method by an adhesive. The third problem is that in the case of the insert molding, the raw material of the magnet is limited to resin materials and therefore it is impossible to mold form a ferromagnetic magnet.
The problems in connection with the above methods become more serious, synergistically, as the outer diameter of the magnet rotor is reduced in order to downsize an electromagnetically-driving device. Particularly, when the diameter of the rotary shaft is made smaller, the area of the bonding face is reduced affecting the securing by an adhesive, securing by press-fit, and insert securing. This poses the problem of ease of detachment owing to, for example, the change in environmental temperature, which leads to a failure during use.
Therefore, it is a major object of the invention to provide a magnet rotor, in which the magnet and driving arm are secured reliably, and not easily separated during use, and which has a simple structure and is easy to machine and assemble. Further, it is an object of the invention to provide an electromagnetically-driving device and a magnet rotor that can be downsized and whose rotary shaft can be made relatively larger in diameter.